Of Boulders and Backflow: how catastrophic megafloods create temporally sustained low-relief surfaces in eastern Himalayan river valleys

Throughout the Quaternary, glacial lake outburst megafloods (discharge >106 m3/s) sourced from the Tibetan Plateau played an important role in shaping the mountainous landscape of the Yarlung-Siang River (YSR). While these floods are famous for their landscape disruptions and intense erosional scars, the depositional legacy of megafloods may be equally important for landscape evolution—creating and maintaining the low-relief terraces that today support human settlement and agriculture in this otherwise rugged terrain. 

Along the main flood pathway, megaflood-transported boulders can generate localized low-relief zones. We observe >100 boulder bars in the YSR that have boulders too large to be moved by annual, or even historic flows. Where boulder bars are deposited in close proximity during a flood, they locally decrease erosion rates, causing a decrease in slope between consecutive bars over a few thousand years. These low slope reaches of the YSR likely experience enhanced deposition during monsoon flooding, potentially forming low-relief terraces adjacent to the YSR as the channel incises and abandons these surfaces. Regardless of genesis, any low-relief surface that is inundated during a megaflood (whether formed by megaflood boulders or through other unrelated processes like landsliding) will experience low bed shear stresses during the flood, causing these surfaces to act as sites of preferential megaflood deposition. Hydraulic modeling demonstrates that once these surfaces exist, subsequent megafloods can easily deposit on them, reinforcing their low-relief character through repeated cycles of inundation and deposition. Along the YSR, there are often co-occurrences of towns/agricultural fields, large boulders, and slackwater deposits. 

Megafloods also generate substantial deposition far beyond the mainstem flood pathway through backflooding of tributaries. Hydraulic modeling shows that backwater inundation likely extended ~60 km up the anomalously low-relief Siyom River from its confluence with the main flood pathway along the YSR. In the Siyom River valley, aggradational terraces preserve distinct sedimentary facies including laminated sands, clays, and peat consistent with slackwater deposition from temporarily impounded waters. Radiocarbon ages from these deposits (10 ka) overlap temporally with inferred occurrences of Tibetan paleolakes, while detrital zircon geochronology reveals that young Tibetan zircons are present in these slackwater deposits—consistent with a megaflood source for these deposits. The low-relief landscape of the Siyom River valley will naturally become a depo-center if inundated by a megaflood, during which low bed shear stresses are produced due to complex backflooding flow dynamics. This deposition will further reinforce the low-relief character of this valley—a valley that is now home to the town of Aalo(ng), the fourth most populous town in the state of Arunachal Pradesh as of 2011. 

Together, these processes demonstrate how catastrophic floods can paradoxically generate geomorphic stability in these low-relief surfaces. While megafloods represent extreme disruptive events, their depositional products create zones of sustained low-relief landscapes that persist for tens of thousands of years, the coincidence of modeled low shear stress zones, depositional facies, and modern agricultural lands reveals this legacy: catastrophic surface processes that disrupt the broader landscape while simultaneously creating resilient, habitable spaces within it.